1. Field of the Invention
The present invention relates to a developing device used in an image formation apparatus such as a copying machine, a laser printer, a facsimile or the like, wherein an electrostatic latent image is electrostatically developed with a two-component type developer.
2. Description of the Related Art
Generally, in an image formation apparatus such as an electrophotographic recording apparatus, the following processes are typically carried out:
a) a uniform distribution of electrical charges is produced on a surface of an electrostatic latent image carrying body; PA1 b) an electrostatic latent image is formed on a charged area of the body surface by an optical writing means such as a laser beam scanner, an LED (light emitting diode) array, a liquid crystal shutter array or the like; PA1 c) the latent image is developed as a visible image with a developer or toner, which is electrically charged to be electrostatically adhered to the latent image zone; PA1 d) the developed and charged toner image is electrostatically transferred from the body to a recording medium such as a cut sheet paper; and PA1 e) the transferred toner image is fixed and recorded on the cut sheet paper by a toner image fixing means such as a heat roller.
Typically, the electrostatic latent image carrying body may be an electrophotographic photoreceptor, usually formed as a drum, called a photosensitive drum, having a cylindrical conductive substrate formed of a metal such as aluminum, and a photoconductive insulating film bonded to a cylindrical surface thereof and formed of an organic photoconductor (OPC), a selenium photoconductor or the like.
As one type of developer, a two-component type developer is well known, which is composed of a toner component (colored fine synthetic resin particles) and a magnetic component (fine magnetic carriers). Usually, a developing device using this type of developer includes a vessel for holding the two-component developer, wherein the developer is agitated by an agitator provided therein. This agitation causes the toner particles and the magnetic carriers to be subjected to triboelectrification, whereby the toner particles are electrostatically adhered to each of the magnetic carriers. The developing device also includes a magnetic roller provided in the vessel as a developing roller in such a manner that a portion of the magnetic roller is exposed therefrom and faces the surface of the photosensitive drum. The magnetic carriers with the toner particles are magnetically adhered to the surface of the magnetic roller to form a magnetic brush therearound, and, by rotating the magnetic roller carrying the magnetic brush, the toner particles are brought to a nip zone or developing zone between the magnetic roller and the drum for development of an electrostatic latent image formed thereon. Namely, the two-component developer is magnetically entrained and brought by the magnetic roller to the developing zone. In the developing process, a developing bias voltage is applied to the magnetic roller so that the toner particles carried to the developing zone are electrostatically attracted only to the latent image, whereby the toner development of the latent image is carried out.
To ensure an even development of a latent image, it is necessary to uniformly regulate an amount of the developer brought to the developing zone by the magnetic roller. To this end, the developing device is provided with a doctor blade disposed in and attached to the vessel to be spaced from the magnetic roller to define a given clearance therebetween, whereby a uniform regulation of the amount of the developer brought by the magnetic roller can be carried out. In assembling the developing device, the doctor blade must be carefully and precisely positioned with respect to the magnetic roller to thereby obtain the given clearance therebetween before a desirable density of development can be ensured. Accordingly, the attachment of the doctor blade to the vessel involves a fine positional adjustment with respect to the magnetic roller, and this fine positional adjustment is troublesome so that the assembly of the developing device is costly.
The developer, from which the toner component is consumed for the development of a latent image, is removed from the magnetic roller by a scraper blade engaged therewith, and is then recovered back to the vessel, whereby a fresh part of the developer held in the vessel can be fed to and entrained by the magnetic roller to thereby maintain a constant density of development during the developing process. Nevertheless, in practice, the constant density of development cannot be maintained during the developing process because the toner component is not uniformly distributed in the developer held in the vessel, due to the recovery of the developer from the magnetic roller to the vessel. Note, this developer has a smaller toner component than the developer held in the vessel because of the consumption of the toner component for the development of the latent image. Of course, the non-uniform distribution of the toner component in the developer causes an uneven development of a latent image.
A developer circulation type device is well known as one type of developing device using the two-component developer, and is provided with an agitator including at least two screw members provided in the vessel and disposed in parallel with each other, and a partition member disposed between the screw members. The screw members are arranged and rotated in such a manner that a part of the developer held in the vessel is circulated between the screw members for the purpose of a uniform distribution of the toner component in the developer. Although the uniform distribution of the toner component in the developer can be obtained, a stable developing process cannot be ensued over a long period because a ratio of the toner component to the developer must be fall within a given range before a proper development can be continuously maintained. Namely, the ratio of the toner component to the developer becomes smaller as a content of the toner component in the developer becomes smaller through the consumption thereof for the development of latent images.
To this end, the developing device may also be provided with a developer supplier so that a toner component can be supplemented to the developer held in the vessel, if necessary, whereby the ratio of the toner component to the developer can be fall within the given range so that an operational life of the developing device can be prolonged. On the other hand, the developer held in the vessel should be periodically exchanged for new developer so that a proper development of a latent image can be continuously maintained, because the magnetic component of the developer is gradually deteriorated to cause an improper development of a latent image. In this case, as much of the old developer as possible should be discharged from the vessel, because the larger the remaining part of the old developer, the sooner the deterioration of the new developer occurs. Also, of course, the exchange of the old developer for the new developer should be carried out as soon as possible. Nevertheless, the conventional developing device fails to rapidly discharge a substantially major part of the old developer from the vessel.